The nuclear hormone receptor superfamily is one of the largest classes of transcription factors and is involved in abundant physiological processes. The 48 members of this family are divided into three classes with class 1 comprised of receptors for androgens (AR), estrogens (ER-α and ER-β), glucocorticoids (GR), progesterone (PR) and mineralocorticoids (MR). Class 2 contains receptors for retinoids, thyroids and vitamin D, while class 3 includes receptors for which ligands are yet to be identified (orphans). Nuclear hormone receptors have an N-terminal domain (NTD) whose function is less characterized, a DNA binding domain (DBD) which is responsible for the binding of receptor to DNA response elements, a hinge region that contains the nuclear localization signal and a ligand binding domain (LBD) to which ligands bind and activate or inhibit receptor action. In addition, there are two activation function domains, one in the NTD (AF-1) and the other in the LBD (AF-2). Due to the high amino acid sequence homology of the DBD, moderate homology and similar secondary and tertiary structural features of the LBD, and common chemical features of steroidal ligands, class I receptors are often capable of binding (i.e., cross reacting) with the ligands of other class I receptors. For example, early studies with AR and ER suggest that the orientation of the steroid in the LBD, with the steroid A-ring in contact with helix-3 and the D-ring in contact with helix-11 residues, is likely to be general for all the steroid hormone receptors.
The androgen receptor (“AR”) is a ligand-activated transcriptional regulatory protein that mediates induction of male sexual development and function through its activity with endogenous androgens. Androgens are generally known as the male sex hormones. The androgenic hormones are steroids which are produced in the body by the testes and the cortex of the adrenal gland or can be synthesized in the laboratory. Androgenic steroids play an important role in many physiologic processes, including the development and maintenance of male sexual characteristics such as muscle and bone mass, prostate growth, spermatogenesis, and the male hair pattern (Matsumoto, Endocrinol. Met. Clin. N. Am. 23:857-75 (1994)). The endogenous steroidal androgens include testosterone and dihydrotestosterone (“DHT”). Testosterone is the principal steroid secreted by the testes and is the primary circulating androgen found in the plasma of males. Testosterone is converted to DHT by the enzyme 5 alpha-reductase in many peripheral tissues. DHT is thus thought to serve as the intracellular mediator for most androgen actions (Zhou, et al., Molec. Endocrinol. 9:208-18 (1995)). Other steroidal androgens include esters of testosterone, such as the cypionate, propionate, phenylpropionate, cyclopentylpropionate, isocarporate, enanthate, and decanoate esters, and other synthetic androgens such as 7-Methyl-Nortestosterone (“MENT”) and its acetate ester (Sundaram et al., “7 Alpha-Methyl-Nortestosterone(MENT): The Optimal Androgen For Male Contraception,” Ann. Med., 25:199-205 (1993) (“Sundaram”). Because the AR is involved in male sexual development and function, the AR is a likely target for effecting male contraception or other forms of hormone replacement therapy.
The human progesterone receptor (PR) occurs as three different isoforms: PR-A, PR-B, and PR-C (Kastner et al., EMBO J 9:1603-1614, 1990; Wei et al., Mol Endo 10:1379-1387, 1996), of which PR-A and PR-B are the most abundant. However, the ratio of PR-A vs. PR-B isoforms is not constant among target tissues, and this can alter the cellular response, because the activity of each isoform can vary.
There are very few compounds that exhibit partial progestin activity under a wide variety of conditions. RU-486, the most commonly used antiprogestin, displays partial agonist activity only under selected conditions. Antiprogestin compounds with partial agonist activity are useful for treating various progestin-regulated diseases and conditions, however, the few known antiprogestins have only limited partial agonist activity, and there remains a need in the art for antiprogestins with broad-range partial agonist activity.
Worldwide population growth and social awareness of family planning have stimulated a great deal of research in contraception. Contraception is a difficult subject under any circumstance. It is fraught with cultural and social stigma, religious implications, and, most certainly, significant health concerns. This situation is only exacerbated when the subject focuses on male contraception. Despite the availability of suitable contraceptive devices, historically, society has looked to women to be responsible for contraceptive decisions and their consequences. Although concern over sexually transmitted diseases has made men more aware of the need to develop safe and responsible sexual habits, women still often bear the brunt of contraceptive choice. Women have a number of choices, from temporary mechanical devices such as sponges and diaphragms to temporary chemical devices such as spermicides. Women also have at their disposal more permanent options, such as physical devices including IUDs and cervical caps as well as more permanent chemical treatments such as birth control pills and subcutaneous implants. However, to date, the only options available for men include the use of condoms and vasectomy. Condom use, however is not favored by many men because of the reduced sexual sensitivity, the interruption in sexual spontaneity, and the significant possibility of pregnancy caused by breakage or misuse. Vasectomies are also not favored. If more convenient methods of birth control were available to men, particularly long-term methods which require no preparative activity immediately prior to a sexual act, such methods could significantly increase the likelihood that men would take more responsibility for contraception.
Administration of the male sex steroids (e.g., testosterone and its derivatives) has shown particular promise in this regard due to the combined gonadotropin-suppressing and androgen-substituting properties of these compounds (Steinberger et al., “Effect of Chronic Administration of Testosterone Enanthate on Sperm Production and Plasma Testosterone, Follicle Stimulating Hormone, and Luteinizing Hormone Levels: A Preliminary Evaluation of a Possible Male Contraceptive, Fertility and Sterility 28:1320-28 (1977)). Chronic administration of high doses of testosterone completely abolishes sperm production (azoospermia) or reduces it to a very low level (oligospermia). The degree of spermatogenic suppression necessary to produce infertility is not precisely known. However, a recent report by the World Health Organization showed that weekly intramuscular injections of testosterone enanthate result in azoospermia or severe oligospermia (i.e., less than 3 million sperm per ml) and infertility in 98% of men receiving therapy (World Health Organization Task Force on Methods And Regulation of Male Fertility, “Contraceptive Efficacy of Testosterone-Induced Azoospermia and Oligospermia in Normal Men,” Fertility and Sterility 65:821-29 (1996)).
A variety of testosterone esters have been developed which are more slowly absorbed after intramuscular injection and thus result in greater androgenic effect. Testosterone enanthate is the most widely used of these esters. While testosterone enanthate has been valuable in terms of establishing the feasibility of hormonal agents for male contraception, it has several drawbacks, including the need for weekly injections and the presence of supraphysiologic peak levels of testosterone immediately following intramuscular injection (Wu, “Effects of Testosterone Enanthate in Normal Men: Experience From a Multicenter Contraceptive Efficacy Study,” Fertility and Sterility 65:626-36 (1996)).
Bone mineral density (BMD) decreases with age in both males and females. Decreased amounts of bone mineral content (BMC) and BMD correlate with decreased bone strength and predispose patients to fracture.
Osteoporosis is a systemic skeletal disease, characterized by low bone mass and deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture. In the U.S., the condition affects more than 25 million people and causes more than 1.3 million fractures each year, including 500,000 spine, 250,000 hip and 240,000 wrist fractures annually. Hip fractures are the most serious consequence of osteoporosis, with 5-20% of patients dying within one year, and over 50% of survivors being incapacitated. The elderly are at greatest risk of osteoporosis, and the problem is therefore predicted to increase significantly with the aging of the population. Worldwide fracture incidence is forecasted to increase three-fold over the next 60 years, and one study estimated that there will be 4.5 million hip fractures worldwide in 2050.
Women are at greater risk of osteoporosis than men. Women experience a sharp acceleration of bone loss during the five years following menopause. Other factors that increase the risk include smoking, alcohol abuse, a sedentary lifestyle and low calcium intake. However, osteoporosis also occurs frequently in males. It is well established that the bone mineral density of males decrease with age. Decreased amounts of bone mineral content and density correlates with decreased bone strength, and predisposes to fracture. The molecular mechanisms underlying the pleiotropic effects of sex-hormones in non-reproductive tissues are only beginning to be understood, but it is clear that physiologic concentrations of androgens and estrogens play an important role in maintaining bone homeostasis throughout the life-cycle. Consequently, when androgen or estrogen deprivation occurs there is a resultant increase in the rate of bone remodeling that tilts the balance of resorption and formation to the favor of resorption that contributes to the overall loss of bone mass. In males, the natural decline in sex-hormones at maturity (direct decline in androgens as well as lower levels of estrogens derived from peripheral aromatization of androgens) is associated with the frailty of bones. This effect is also observed in males who have been castrated.
Muscle wasting refers to the progressive loss of muscle mass and/or to the progressive weakening and degeneration of muscles, including the skeletal or voluntary muscles, which control movement, cardiac muscles, which control the heart (cardiomyopathics), and smooth muscles. Chronic muscle wasting is a chronic condition (i.e. persisting over a long period of time) characterized by progressive loss of muscle mass, weakening and degeneration of muscle.
The loss of muscle mass that occurs during muscle wasting can be characterized by muscle protein degradation by catabolism. Protein catabolism occurs because of an unusually high rate of protein degradation, an unusually low rate of protein synthesis, or a combination of both. Muscle protein catabolism, whether caused by a high degree of protein degradation or a low degree of protein synthesis, leads to a decrease in muscle mass and to muscle wasting.
Muscle wasting is associated with chronic, neurological, genetic or infectious pathologies, diseases, illnesses or conditions. These include muscular dystrophies such as Duchenne muscular dystrophy and myotonic dystrophy; muscle atrophies such as post-polio muscle atrophy (PPMA); cachexias such as cardiac cachexia, aids cachexia and cancer cachexia, malnutrition, leprosy, diabetes, renal disease, chronic obstructive pulmonary disease (COPD), cancer, end stage renal failure, sarcopenia, emphysema, osteomalacia, HIV infection, AIDS, and cardiomyopathy.
In addition, other circumstances and conditions are linked to and can cause muscle wasting. These include chronic lower back pain, advanced age, central nervous system (CNS) injury, peripheral nerve injury, spinal cord injury, chemical injury, central nervous system (CNS) damage, peripheral nerve damage, spinal cord damage, chemical damage, burns, disuse deconditioning that occurs when a limb is immobilized, long term hospitalization due to illness or injury, and alcoholism.
An intact androgen receptor (AR) signaling pathway is crucial for appropriate development of skeletal muscles. Furthermore, an intact AR-signaling pathway increases lean muscle mass, muscle strength and muscle protein synthesis.
Muscle wasting, if left unabated, can have dire health consequences. For example, the changes that occur during muscle wasting can lead to a weakened physical state that is detrimental to an individual's health, resulting in increased susceptibility to bone fracture and poor physical performance status. In addition, muscle wasting is a strong predictor of morbidity and mortality in patients suffering from cachexia and AIDS.
New innovative approaches are urgently needed at both the basic science and clinical levels to develop compounds which are useful for a) male contraception; b) treatment of a variety of hormone-related conditions, for example conditions associated with Androgen Decline in Aging Male (ADAM), such as fatigue, depression, decreased libido, sexual dysfunction, erectile dysfunction, hypogonadism, osteoporosis, hair loss, anemia, obesity, sarcopenia, osteopenia, osteoporosis, benign prostate hyperplasia, alterations in mood and cognition and prostate cancer; c) treatment of conditions associated with ADIF, such as sexual dysfunction, decreased sexual libido, hypogonadism, sarcopenia, osteopenia, osteoporosis, alterations in cognition and mood, depression, anemia, hair loss, obesity, endometriosis, breast cancer, uterine cancer and ovarian cancer; d) treatment and/or prevention of chronic muscular wasting or sarcopenia; e) decreasing the incidence of, halting or causing a regression of prostate cancer; f) oral androgen replacement and/or other clinical therapeutic and/or diagnostic areas.
A wide variety of diseases and/or conditions are affected by hypogonadism, and catabolic effects, including kidney disease, central nervous system injuries, burns and chronic wounds.
In the United States (US), there is a rising incidence and prevalence of kidney failure. The number of patients enrolled in end-stage renal disease (ESRD) Medicare-funded programs has increased from approximately 10,000 beneficiaries in 1973 to 86,354 in 1983 and to 431,284 as of Dec. 31, 2002. In 2002 alone, 100,359 patients entered the US ESRD program. Chronic kidney disease (CKD) is a precursor to ESRD and occurs when the kidneys are not able to adequately remove wastes from the body. CKD is a slowly progressing disease, in which diabetes, hypertension and anemia may be co-morbid conditions.
CKD is diagnosed using a staging system that demonstrates the amount of kidney function available (stage 1=normal kidney function) and patients often do not present symptoms in the early stages. Stage 5 of CKD is ESRD, which is a complete or near complete failure of the kidneys and usually occurs when kidney function is less than 10% of baseline.
Accompanying symptoms associated with ESRD include hypogonadism, involuntary weight loss, fatigue and others.
Burns result in a testosterone reduction, nitrogen level reduction and a reduction in bone mineral density (BMD), which may persist even as long as one year following the injury and is associated with impaired wound healing, increased infection risks, erosion of lean body mass, hampered rehabilitation, and delayed reintegration of burn survivors into society. Catabolic effects initiated as a result of the burn lead to significant involuntary weight loss, further compounding the problem.
Spinal cord injuries (SCI) may result in the alteration central neurotransmitter secretion or production, which in turn may cause a hypothalamus-pituitary-adrenal axis dysfunction, leading to decreases in testosterone and other hormone levels. SCI or other acute illness or trauma characteristically includes heightened catabolism in conjunction with the lowered anabolic activity resulting in a condition that is prone to loss of lean body tissue. As long as the catabolic process goes uninterrupted, disturbed nutrient utilization will continue. The effects of the loss of lean body mass include the development of wounds and impaired healing mechanisms. Because of poor nutrition and protein combined with immobilization, patients with spinal cord injury are at high risk for bed sores.
Chronic wounds may be caused by any number of conditions, including diabetes, circulatory problems, immobilization and others. Compounding the problem, for example in diabetes, is the presence of neuropathy, which increases the risk of foot ulceration.
While there are many treatments and therapies for these conditions, none are ideal. Since the androgen receptor (AR) signaling pathway has been shown to increase lean muscle mass, muscle strength and muscle protein synthesis, and since hypogonadism accompanies these conditions, molecules targeting the AR signaling pathway may be useful in treating these diseases and/or conditions.